Screw thread cutting apparatus and method

ABSTRACT

A thread cutting milling tool comprises a shank with a working portion formed with a recess for receiving a thread cutting insert. The thread cutting insert has one edge formed with a line of thread-form teeth of identical profile. The thread cutting teeth project laterally for cutting engagement with a work-piece while the tool spins about its axis. The tool also orbits within or about a work piece, and in one orbit of the tool in a helical cutting path relative to the work piece a number of complete turns of the screw-thread are formed which is equal to the number of teeth of the insert in engagement with the work-piece. A secondary cutter carried on the thread cutting milling tool provides for formation of a pilot portion or counter bore on the work piece at an end of the screw thread. The pilot portion or counter bore provides for alignment of a threaded member which is to engage the screw thread. Also, the secondary cutter provides for formation of a blunt or scalloped full-thread starting end for the screw thread at the pilot portion. The blunt starting end for the screw thread is robust, resists damage, and resists cross threading.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Divisional Application of U.S. Ser. No.10/877,854, filed Jun. 25, 2004, now U.S. Pat. No. ______, granted______.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns the cutting of screw-threads. More particularly,this invention relates to a screw thread cutting apparatus and methodfor cutting screw threads on a surface, which surface may be either aninterior surface (i.e., female screw threads) or exterior (i.e., malescrew threads), and which may be a straight cylindrical surface or atapering surface. Still more particularly, the present invention relatesto such a screw thread cutting apparatus and method in which thestarting end of the first screw thread is spaced away from an adjacentend of the threaded surface to provide a pilot portion or counter boreportion. Further, the starting end of the first screw thread is blunt orscalloped, so as to provide a jam-resistant start for the screw thread,and to avoid cross threading as well.

2. Brief Description of Related Technology

The use of a screw thread cutting tool including a milling cutter with ashank defining a recess in which is received a replaceable mutli-tooththread cutting insert is well known and conventional. These threadcutting tools are employed in computer numerically controlled (CNC)milling machines to cut both internal and external threads, whichthreads may be of straight (i.e., cylindrical) shape, or may be tapered(i.e., conical shape). The cutting of such screw threads on or within awork piece conventionally creates a thread form that runs right up tothe end of an externally threaded part, or right up to the open end of abore defined within an internally threaded part.

Conventional technology of the type described immediately above may beseen in U.S. Pat. Nos. 4,913,604; and 5,098,232

With the conventional technology, after the performance of an interioror exterior thread cutting operation, if it is desired to provide astarting pilot portion on a work piece having a male screw thread, or astarting counter bore portion within a work piece having a femalethreaded bore, then in each case the provision of such starting portionsrequires the performance of a second operation. That is, a secondcutting operation must be performed using a tool to form the counterbore or pilot portion on the work piece. Further, when such a startingportion is provided on a male or female threaded part, the starting endof the first thread adjacent to the starting portion is most usuallyreduced to a thin edge, or to a fine-dimension. That is, at its startingend, the thread form tapers from a full thread shape to essentially afine edge or toward a zero thickness. Such a fine starting end of afirst thread also has a low thread form height, so that it is easilycross threaded in the event that threaded parts are not perfectlyaligned when it is attempted to screw them together. That is, such afine-dimension or thin starting end for a screw thread is fragile and ishighly subject to damage. Such damage may cause jamming of threadedparts when assembly of these parts is attempted. Further, afine-dimension or thin starting end on the first thread tapers to a lowheight which is easily crossed over by a matching thread and promotescross threading of screw threaded parts if an attempt is made toassemble them with imperfect alignment. Such cross threading when itoccurs can ruin the screw threads, and can possibly make the screwthreaded parts unusable as well.

SUMMARY OF THE INVENTION

An object for this invention is to reduce or eliminate one or more ofthe deficiencies of the conventional technology.

Further, an object for this invention is to provide a thread cuttingmilling tool and method which provides a starting pilot at the start ofa male or female thread cut with the tool.

A further object for this invention is to provide a thread cuttingmilling tool and method as described above, and in which the startingpilot leads to a starting or first thread having a blunt or scallopedend, such that the starting pilot assists in aligning threaded parts tobe assembled, and the blunt starting or first thread is both robust andresistant to damage, and also resists cross threading.

The present invention provides according to one aspect, a thread cuttingtool with a recess in which is received a multi-tooth thread cuttinginsert formed with a line of teeth which are identical to each other andwhich have a profile complementary to that of the finished tooth of thescrew-thread to be cut. In use of this thread-cutting tool, the tool ismounted on the spindle of a CNC milling machine for rotation about theaxis of the spindle and the spindle is simultaneously moved in a pathcentered on the axis of the surface of the work-piece which is to bescrew-threaded. The work-piece is mounted on a stationary work table.The thread cutting tool includes a secondary cutter spaced from andhaving a selected axial and rotational orientation relative to thethread cutting insert. The secondary cutter is so oriented relative tothe thread cutting insert that a starting pilot is cut by the secondarycutter when it engages the work piece. Further, the secondary cutterscallops or blunts the end of the starting or first thread of thefinished screw thread which is adjacent to the starting pilot.

If the inventive thread cutting milling tool is to cut a straight screwthread, then the line of thread cutting teeth of the insert is disposedparallel to the axis of the work-piece on or in which the thread is tobe cut, so that where the surface is cylindrical the line of teethextends parallel to the axis of the shank of the tool. On the otherhand, where the surface to be screw threaded is conically inclined(i.e., to make a tapering thread) the line of thread cutting teeth ofthe thread cutting milling tool is similarly inclined to the axis of theshank of the tool.

Where the threaded surface is cylindrical, the radius of the cuttingtool path as it orbits or gyrates in engagement with the work piece isconstant; but where the threaded surface is conically inclined theradius of the cutting tool path (i.e., its orbital and helical path ofengagement with the work piece) is varied continuously to follow thedesired profile of the tapered thread surface.

In one aspect the invention provides a thread-cutting tool comprising anelongate shank having a lengthwise axis; a recess or pocket formed inthe shank for receiving a multi-tooth thread cutting insert, and astarting thread removal cutter projecting laterally of the threadcutting tool such that this starting thread removal cuttingsubstantially removes the first thread adjacent to an end of the workpiece.

Removal of the first thread adjacent to an end of the work piece (i.e.,adjacent to an end of the thread on an externally threaded part, oradjacent to the opening of a threaded bore within an internally threadedpart) to provide a starting portion (i.e., a pilot or counter boreportion) makes alignment and threading engagement of threaded parts withone another very much easier. The removal of the first thread on aexternally threaded part provides a cylindrical pilot feature which isfirst inserted into a threaded bore of an internally threaded part, andguides the externally threaded part toward threading engagement.Similarly, the removal of the first thread of a female threaded partprovides a counter bore starting pilot portion, which may receive andassist in aligning a male threaded part.

Still further, a blunted or scalloped end of the first or startingthread on a threaded part makes jamming of threaded parts upon assemblymuch more unlikely. Conversely, such a blunted or scalloped starting orfirst thread on a threaded part or work piece provides a much morerobust, damage resistant, and cross threading resistant part.

Additional objects and advantages of the present invention will beapparent in view of the following description of preferred exemplaryembodiments of the invention, taken in conjunction with the followingdrawing Figures, in which like reference numerals indicate like featuresthroughout the several views.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a side elevation view of a thread cutting milling toolembodying the present invention;

FIG. 2 is a side elevation view similar to FIG. 1, and having a portionof the structure of the thread cutting milling tool broken away forclarity of illustration;

FIG. 3 is a side elevation view similar to FIGS. 1 and 2, with the viewbeing taken along line 3-3 of FIG. 2, so that the viewer is lookingperpendicularly to the direction of view seen in FIG. 2;

FIGS. 4 and 5 are sequential views of the thread cutting milling toolseen in FIGS. 1-3 being employed to form an external thread on a workpiece and to providing a thread starting pilot feature on the free endof the work piece;

FIGS. 6 and 7 are respective side elevation views of a thread cuttingmilling tool according to an alternative embodiment of the presentinvention, and show opposite sides of the thread cutting milling tool,and FIG. 7 also shows a step in the method of employing the threadcutting milling tool seen in these Figures to simultaneously form twoscrew thread portions of the same hand on opposite ends of an elongatework piece, and to also simultaneously form a thread starting pilotfeature on each of the thread portions;

FIG. 8 is an enlarged fragmentary view of a screw threaded portion of awork piece having screw threads formed in accord with the first orsecond embodiment of this invention;

FIG. 8A is a still more greatly enlarged fragmentary portion of thescrew threaded portion of the work piece seen in FIG. 9 and is takenadjacent to a starting pilot feature of this work piece; and

FIGS. 9 and 10 are respective illustrations of sequential steps in themethod of using a thread cutting milling tool according to the presentinvention in order to form a thread portion adjacent to the upper end ofa tubular work piece.

DETAILED DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS

Referring to FIGS. 1, 2, and 3 in conjunction, a thread cutting tool orthread cutting milling tool 10 embodying this present invention includesan elongate shank 12 defining a recess or pocket 14, in which isreceived a multi-tooth thread cutting insert 16. The elongate shank 12includes a working portion 12 a on which the pocket 14 is formed, and anengagement portion 12 b which is chucked by the CNC milling machine. Aflat 12 c provided on the engagement portion 12 b provides for positivedriving of the tool 10 by the CNC milling machine.

In this particular embodiment of the tool 10, the pocket 14 and threadcutting insert 16 are each trapezoidal in plan view, as is seen in FIGS.1 and 2. However, the present invention is not limited to use of athread cutting insert of any particular form or shape. As is seen inFIG. 3, the pocket 14 has a flat bottom surface 18, and a threaded bore20 opens to this flat bottom surface 18. A thread cutting insert 16 isreceived into the recess 14, and is shaped in plan view (as is seen inFIGS. 1 and 2) to seat closely and securely in this pocket 14. A sockethead cap screw 22 of button head configuration is received through ahole 24 defined by the thread cutting insert 16 and threadably engagesinto the bore 20. Thus, the insert 16 is securely held in the pocket 14by cap screw 22. The insert 16 defines plural radially outwardlyextending thread cutting teeth 26.

As those ordinarily skilled in the pertinent arts will appreciate, ifthe tool 10 is intended to cut straight threads (as is tool 10illustrated in the FIGS. 1-5) then the teeth 26 will be arrayed parallelto the axis of the shank 12. On the other hand, in the event that thethread cutting milling tool 10 is intended to cut tapered threads (i.e.,tapered pipe threads, for example) then the pocket 14 is disposedrelatively angulated relative to the axis of the shank 12, so that thecutting insert 16 is similarly arranged to present thread cutting teeth26 arrayed along a line that is angulated relative to the axis of shank12, according to the taper desired to be provided on the tapered threadscut with the tool 10.

Those ordinarily skilled in the pertinent arts will also appreciate thatthe features of the thread cutting milling tool 10 described to thispoint are conventional, and are within the scope of the conventionaltechnology. Also, those ordinarily skilled will appreciate that thethread cutting milling tool 10 may be employed to cut male or femalethreads. Because the principles and operation of the thread cuttingmilling tool 10 are easily transposed from cutting male screw threads tocutting female screw threads, the disclosure below addresses only thecutting of male screw threads. In the case of use of the thread cuttingmilling tool to cut female screw threads, then the relative positions ofthe minor and major diameters of the screw thread so cut are reversedrelative to the center axis of the screw thread. Further to the above,it is seen in FIGS. 1-3 that the thread cutting milling tool 10 alsocarries a secondary radially projecting cutter 28 spaced a selecteddistance (i.e., in axial distance, or possibly both in axial androtational orientation about the axis of shank 12) from the threadcutting insert 16. The secondary cutter 28 is disposed in a transversebore 30 provided in the shank 12. A portion of the transverse bore 30provides a screw thread 30 a and a set screw 32 received into thethreaded bore portion 30 a engages the secondary cutter 28 to allowadjustment of the extent of outward radial projection or protrusion ofthis secondary cutter 28. The secondary cutter 28 is secured in aselected position of radially outward protrusion by a lock screw 34, inthe form of a set screw threadably received into an intersecting bore 36defining a screw thread 38. That is, an end of the lock screw 34 bearsagainst a shank portion 40 of the secondary cutter 28.

As is seen in FIGS. 1 and 2, the secondary cutter 28 preferably includesa pair of opposite radially extending side cutting edges 42, 44. Also,the secondary cutter 28 defines an axially extending cutting edge 46extending between radially outer terminations of the side cutting edges42, 44. As a result, the radially outwardly protruding portion of thesecondary cutter 28 as seen in FIGS. 1 and 2 has a square or rectangularshape. As FIG. 3 shows, the shank portion 40 of the secondary cutter 28may be round in transverse sectional view, although the invention is notso limited.

FIGS. 4 and 5 illustrate sequential steps in the method of utilizing aCNC milling machine (not seen in the drawing Figures) and a threadcutting milling tool 10 as described by reference to FIGS. 1-3, to forman external (i.e., male) screw thread 50 on the outside of a stationarycylindrical work piece 52. In this respect, FIGS. 4 and 5 are merelydescriptive, and the tool 10 could as well be used to cut a female screwthread within a bore of a work piece. Those ordinarily skilled in thepertinent arts will understand that concepts and teachings of cuttingmale or female screw threads with a CNC milling machine and by use of atool 10 embodying this invention are equivalents of one another.

In preparation to this screw thread cutting operation (which is shown inprogress in FIG. 4), and while the tool 10 is still spaced radially fromengagement with the work piece 52, the tool 10 is spun up or rotated athigh speed about its axis by the CNC milling machine, as is indicated onFIG. 4 by arrow 54. The tool 10 is then advanced radially intoengagement with the work piece 52, as is indicated by arrow 56, so thatthe thread cutting insert 16 engages the work piece 52. Simultaneously,the CNC milling machine gyrates or orbits the tool 10 about the workpiece 52, while maintaining the desired degree of engagement of thethread cutting insert 16 with the work piece 52, as is indicated byarrow 58.

The result is that the tool 10 spins on its axis, gyrates or orbitsabout the work piece 52 with the thread cutting insert 16 engaging thework piece, and advances axially according to the pitch of screw thread50 with each gyration. So, the cutting of the screw thread 50 advancesfrom the free end 62 of the work piece along the length of this workpiece (as is seen in FIG. 4) according to the length of the screw thread50 desired. In the event that the tool 10 is used to cut a tapered screwthread, in addition to the angulation of the pocket 14 and threadcutting insert 16, the tool 10 will be moved slightly outward radiallyas it progresses about (i.e., gyrationally or orbitally) and axiallyalong the work piece 52. This slight radial movement is coordinated withthe taper angle of the screw thread so that the required increase inscrew thread diameter from starting end 62 to the finish of the screwthread 50 is achieved.

As FIG. 5 illustrates, when the desired length of screw thread 50 isnearly finished being cut on work piece 52, the secondary cutter 28 alsoengages the work piece 52. This results from the axial relative motionof the cutter 10 along the work piece 52, as is indicated by arrow 60.As the secondary cutter 28 engages the work piece 52, it removessubstantially the first thread form of screw thread 50 (i.e., the firstwrap of thread from end edge 62 axially and helically about the workpiece 52 through substantially one thread pitch dimension. That is, thesecondary cutter 28 removes the screw thread form axially and throughsubstantially 360 degrees of helical motion about the work piece 52. Asis seen in FIG. 5, the result is a cylindrical pilot portion 64 on workpiece 52. The axially extending end cutting edge 46 of the secondarycutter 28 is disposed relative to the thread cutting insert 16 so thatthe pilot portion 64 has a diameter just slightly less than the minordiameter of the screw thread 50 formed by the thread cutting insert 16.

Further, it is desired that the starting end (or end of the firstthread) of screw thread 50 immediately adjacent to pilot portion 64 notstart with a fine-edged partial thread, but that this screw thread startwith substantially a full thread form. Because the tool 10 carries thesecondary cutter 28 in a radially outwardly disposed orientation, and atthe completion of a thread cutting operation this tool is disengagedfrom the work piece 52 by the CNC milling machine effecting a quickradially outward relative motion (indicated by arrow 66) at thecompletion of the cutting of screw thread 50, the starting end 50 a ofscrew thread 50 immediately adjacent to pilot portion 64 will inherentlyhave a scalloped or concave blunt end surface which is of substantiallyfull screw thread form. This is the case because the spinning of cutter10 is very fast in relation to the quick outward radial motion indicatedby arrow 66, so that the cutting surface 46 of cutter 28 effects aradius of cut on the end of the screw thread 50 which is about equal tothe effective cutting diameter of the edge 46 of cutter 28 as it spinson tool 10.

Further, in order to achieve this desired blunt or scalloped full-threadform start 50 a of the screw thread 50 adjacent to pilot portion 64, theaxial spacing and rotational orientation of the secondary cutter 28 onshank 12 relative to the position of the thread cutting insert 16 issuch that an axial spacing of an integer number of thread pitches minusabout one-half (½) a thread pitch is disposed axially between the peakof a thread form 50 on work piece 52 formed by the cutting teeth oncutter 16, and the edge 42 of cutter 28. That is, any one of the peaksof the thread form 50 on work piece 52 formed by the cutting teeth ofthe cutter 16 is an integer number of thread pitches minus aboutone-half a thread pitch dimension axially from the cutting edge 42 ofthe secondary cutter 28. This axial relationship is illustrated in FIG.4 by the indicated axial spacing arrow and the equation: “xP−½P”, inwhich “x” is an arbitrary integer, and “P” is the screw thread axialpitch dimension.

Stated differently, the result of the axial and rotational relationshipof the thread cutting insert 16 and secondary cutter 28 is that thecutting edge 42 of the cutter 28 is about centered on the valley betweenadjacent screw threads 50, or on the minor diameter of the thread formof screw thread 50, and as the tool 10 advances axially while thecutting of screw thread 50 is completed, this cutting edge 42 and axialedge 46 progressively remove the first wrap of thread form starting fromthe end edge 62, forming pilot portion 64, and progressing helically tothe blunt thread end 50 a. So, as this cutting edge 42 of the secondarycutter 28 removes the first wrap of thread form adjacent to the pilotportion 64, a scalloped or rather blunt substantially full thread formis maintained at the start 50 a of the screw thread 50 adjacent tostarting pilot portion 64.

As described immediately above, such will be the axial relationship ofthe thread cutting insert 16 and the secondary cutter 28 if they liealong substantially the same longitudinal line of shank 12 (as is seenin FIGS. 1 and 2, and as is best illustrated in FIG. 3). By carefulconsideration of FIG. 3 it can be appreciated that the cutters 16 and 28are substantially aligned with one another along the length and on thesame side of the shank 12. In the event that the positions of secondarycutter 28 and thread cutting insert 16 are relatively rotated along theshank 12, then the desired axial spacing between these two cutters inorder to form a blunt-start screw thread 50 a at the beginning of screwthread 50 will be adjusted in view of the rotational orientation ofthese two cutters. That is, a 180 degree orientation of these cuttersrelative to one another would require an axial relative spacing of aninteger number of thread pitches between the any one of the peaks of thethread form 50 and the cutting edge 42 of the cutter 28. The importantfeature is that the cutting edge 42 of the secondary cutter be disposedabout centered on the valley of the thread form cut by the threadcutting insert 16. In this way, the cutting edge 46 will remove thethread form and also scallop the starting end 50 a of the screw thread50 adjacent to pilot portion 64.

Other possible axial and rotational relative orientations between thethread cutting insert 16 and secondary cutter 28 on shank 12 are, ofcourse, possible. However, the required relationship of the cutters 16and 28 in order to achieve a blunt, or full-thread start 50 a for thescrew thread 50 at pilot portion 64 is clear in view of the above, andis within the skill of those ordinarily skilled in the art who have nowreceived the advantages of this disclosure.

FIGS. 6 and 7 provide side elevation views of the opposite sides of analternative embodiment of a thread cutting milling tool according tothis invention. Because the alternative embodiment of FIGS. 6 and 7 hasmany features in common with the first embodiment described inconnection with FIGS. 1-5, features of the alternative embodiment thatare the same as or analogous in structure or function to those describedand depicted earlier are referenced with the same numeral used above,and increased by one-hundred (100).

Viewing now FIGS. 6 and 7 in conjunction with one another, it is seenthat a thread cutting tool or thread cutting milling tool 110 embodyingthis present invention includes an elongate shank 112 defining a pair ofspaced apart recesses or pockets 114 a and 114 b. Received into each oneof the pair of pockets 114 a and 114 is a respective one of a pair ofmulti-tooth thread cutting inserts 116 a and 116 b.

A respective one of a pair of socket head cap screws 122 a and 122 beach of button head configuration is received through a hole 124 definedby the thread cutting inserts 116 a and 116 b. These cap screws 122 aand 122 b and threadably engage into respective threaded bores (not seenin FIGS. 6 and 7 in order to secure the thread cutting inserts 116 a and116 b in the recesses 114 a and 114 b.

Each of the inserts 116 a and 116 b defines plural radially outwardlyextending thread cutting teeth 126 a and 126 b. As was explained above,the pockets 114 a and 114 b may be arranged to present the threadcutting inserts 116 a and 116 b to cut either straight or taperedthreads.

The thread cutting milling tool 110 also carries a pair of secondaryradially projecting cutters 128 a and 128 b each spaced along the shank112 (i.e., spaced axially and possibly rotationally as well) a selecteddistance from the adjacent thread cutting insert 116 a and 116 b. Eachone of the pair of secondary cutters 128 a and 128 b are disposed in arespective pair of transverse bores (indicated by arrowed numerals 130 aand 130 b) provided in the shank 112, and are there adjustable for depthin the same way described above with respect to the first embodiment, aswell as being secured by a respective set screw or lock screw 134 a or134 b.

FIG. 7 also illustrates a step in the method of utilizing a CNC millingmachine (not seen in the drawing Figures) and a thread cutting millingtool 110 as described by reference to FIGS. 6 and 7 to simultaneouslyform a pair of spaced apart external (i.e., male) screw thread portions150′ and 150″ on the outside of a stationary cylindrical work piece 152.In this case, the work piece 152 is tubular and is carried on a collet(not seen in FIG. 8) extending upwardly above a base member 152 a.

As is seen in FIG. 7, the screw threads 150′ and 150″ are formed whilethe tool 110 is rotated by a CNC milling machine (not seen in thedrawing Figures) at high speed about the axis of shank 112 (as isindicated on FIG. 7 by arrow 154), and is advanced radially intoengagement with the work piece 152 (indicated by arrow 156) and issimultaneously gyrated or orbited about the work piece 152 (i.e., whilemaintaining the desired degree of engagement of the thread cuttinginserts 116 with the work piece 152), and is simultaneously advancedaxially according to the pitch of screw threads 150′ and 150″ (whichhave the same pitch and same handedness) with each gyration (indicatedby arrows 158 and 160).

So, at one end of the work piece 152, the cutting of the screw thread150′ or 150″ advances from the respective free end 162 a or 162 b of thework piece along the length of this work piece, while at the other endof the work piece, the thread cutting starts at a point intermediate ofthe ends of the work piece and spaced from the adjacent free end (162 aor 162 b), and progresses toward the respective free end. In otherwords, viewing FIG. 7, the relative axial motion of the thread cuttingtool 110 along work piece 152 during thread cutting may be relativelyupwardly or downwardly. In each case, screw threads 150′ and 150″ of thesame hand (i.e., left handed or right handed screw threads) aresimultaneously formed on the opposite end portions of the work piece152.

Also, each screw thread portion 150′ and 150″ is provided with a pilotportion 164 a and 164 b by action of the secondary cutters 128 a and 128b. Still further, the starting thread at each pilot portion 164 a and164 b will have a blunt or scalloped full-thread form surface 150 a, 150b. The formation of such a blunt starting thread end 150 a, 150 b at oneend of the work piece is easy to understand in view of the descriptionof the first embodiment above. That one end is the one having the samesense of tool movement described above with respect to the firstembodiment, in which the relative tool movement is from the free end ofthe work piece toward a screw thread completion spaced from this freeend. In the same sense as described above for the first embodiment, thesecondary cutter (128 a or 128 b) of the tool 110 will form the pilotportion (164 a or 164 b) and will scallop the starting end of the firstscrew thread.

However, formation of a blunt starting thread form end 150 a or 150 b atthe opposite end of the work piece is accomplished by the secondarycutter 128 a or 128 b first making a cut into the work piece, andproceeding helically toward the adjacent free end of the work piece toform the pilot portion. Then the thread cutting insert working at thatend of the work piece advances also toward the adjacent free end(cutting screw threads as it progresses) and encounters the starting cutmade by the secondary cutter. Because of the axial and rotationalrelationship of the thread cutting inserts and secondary cutters asdescribed above, in this case the screw thread is cut with an alignmentsuch that the starting cut made by the secondary cutter becomes the endof the thread form when the screw thread is cut at that location by thecutter 116. Again, the secondary cutter 128 a or 128 b provides a bluntor scalloped full-thread starting end for the thread form cut by theadjacent cutter 116 a or 116 b.

Stated differently, at one end of the work piece 152, the screw threadadjacent to the end edge 162 a or 162 b is cut first, followed byformation of the pilot portion 164 and scalloping of the starting screwthread end. At the other end of the work piece 152, the pilot portion162 a or 162 b is cut first (while the thread cutting insert is workingat some distance from the starting pilot portion), and the threadleading to this pilot portion is formed next, ending at a surface 150 aor 150 b of the pilot portion which was cut earlier by the secondarycutter 128 a or 128 b. Because the thread cutting inserts and secondarycutters have the desired axial and rotational relationship, thescalloped starting end of the thread form can be cut before or after thecutting of the screw thread.

Still further, it is possible with the thread cutting milling tool 110of FIGS. 6 and 7 to simultaneously cut tapered threads of the same handon opposite ends of a work piece, but with the screw threads so cuthaving opposite directions of taper. And, the work piece is alsoprovided with a respective pilot portion at which the tapered threadportion starts with a blunt full-thread. This unique combination offunctions is possible using a thread cutting milling tool according tothe present invention, by selecting the lengths of the thread cuttinginserts 116 a and 116 b to be substantially equal in length to thelength of each screw thread portion 150′ and 150″ minus a single threadpitch dimension. Particularly, the thread cutting inserts can be onethread pitch less in length than the number of tapered screw threadsdesired to be cut onto a work piece. Thus, the screw thread cuttingmilling tool 110 will make a single helical orbit about the work piece152, with no radial motion to account for screw thread taper. The taperof the screw threads so cut is achieved simply by the angulation of thethread cutting inserts 116 a and 116 b on the tool 110. Further, thesecondary cutters 128 a and 128 b are rotationally positioned on theshank 112 such as to provide room for both the thread cutting inserts116 a and 116 b and the adjacent secondary cutters 128 a and 128 b ateach end of the working portion of shank 112.

Turning now to FIGS. 8 and 8A an additional embodiment of the inventionis there illustrated. Again, because this embodiment has many featuresin common with the first two embodiments, features which are the same orwhich are analogous in structure or function to those depicted anddescribed above are indicated with the same reference numeral usedabove, and increased by two-hundred (200). Viewing FIGS. 8 and 8A, it isseen that a work piece 252 has screw threads 250 cut thereon by themethod of this invention and using a tool as described above. However,it will be noted also that the first full wrap of the screw thread 250has both a blunt starting surface 250 a, as well as a slight undercut(indicated by arrowed numeral 66) which extends axially about 0.005 inchfrom the center of the thread form at its minor diameter and into thenext adjacent full thread. This undercut 66 is achieved by having thesecondary cutter 228 positioned about 0.005 inch closer to the threadcutting insert 216 than would be indicated by the relationship set outabove. That is, instead of the secondary cutter 228 being positionedwith the cutting edge 242 at an integer number of thread pitches minusone-half thread pitch, as described above; the secondary cutter 228 isso positioned with an additional 0.005 inch of spacing subtracted fromthe integer number of thread pitches (minus one-half thread pitch) whichspace the thread cutting insert (not seen in the drawing Figures) fromsecondary cutter 228. Thus, the secondary cutter 228 fully removes thethread form radius which is formed at the minor diameter of the screwthread 250 by the thread cutting insert. That is, a close considerationof standard thread forms will reveal that the minor diameter is definednot by a sharp angular intersection of adjacent thread surfaces, but bya blending radius. This blending radius is removed by the embodiment ofFIGS. 8 and 8A in the pilot portion 264 and up to the blunt starting end250 a. As a result, ease of assembly of the work piece 250 with anotherscrew threaded part is better assured.

Turning now to FIGS. 9 and 10 in conjunction with one another, a fixture152 a (as was alluded to above) and including an expansible colletstructure 70 is carried by a base plate 72 having a cylindrical bushingmember 74 extending upwardly on the base plate. The bushing 74 guides adraw bolt 76 which is forcefully reciprocable relative to the baseplate, as is indicated by the associated double headed arrow on FIG. 9.The draw bolt 76 includes a head portion 78 having a forcing conesection 78 a on its underside, and bearing against matching conicalsurfaces 70 a of the collet 70. At the lower end of the collet 70,opposed conical surfaces 70 a are engaged by a forcing washer 80, whichsurrounds the pull bolt 76 and rests atop of the bushing 74. A stripperplate 82 is carried above the base plate 72, and carries a tubularstripper boss 84 surrounding the bushing 74. The stripper plate 82 andstripper boss 84 are reciprocable vertically relative to the base plate72, as is indicated by the associated double headed arrow of FIG. 9. Inthe relative positions of the stripper plate 82 and base plate 72illustrated in FIG. 9, the top surfaces of the bushing 74 and of thetubular stripper boss are coextensive. Also as is seen in FIG. 9, thetop surface of the bushing 74 provides a surface supporting the collet70, while the top surface of the stripper boss 84 provides a surface forsupporting a work piece 152.

As is indicated in FIG. 9, when the stripper plate 82 is in engagementwith the base plate 72 and a work piece 152 is engaged over the collet70, a forceful downward movement of the draw bolt 76 expands the collet70 and locks the work piece 152 in place. As thus locked in place, thework piece 152 may be screw threaded on its radially outer surfaces, aswas explained above with respect to the use of the thread cutting tool10. In FIG. 9, this screw thread cutting operation is illustrated asalready having taken place on the work piece 152.

Now, on the other hand, at the completion of a screw thread cuttingoperation, as is indicated in FIG. 10, an upward relative motion of thedraw bolt 76 allows the collet 70 to contract radially a small amount,which is enough to release the locking hold of the collet 70 on the workpiece 152. Subsequently, the stripper plate 82 is forcefully movedslightly upwardly relative to the base plate 72, so that the work piece152 is raised a fraction of the length of the collet member 70, inpreparation for the work piece 152 being removed from the fixture 152 a.

While the invention has been described with reference to the foregoingpreferred exemplary embodiments, many changes and modifications may bemade thereto which fall within the scope of the appended claims.

1. A method of utilizing a single milling tool in a single operation tosimultaneously provide on a work piece both a cut screw thread and athreadless portion adjacent to one end of the cut screw thread, whilealso providing a blunt starting thread of full-thread-form at saidthreadless portion, said method including steps of: providing a threadcutting milling tool having an elongate rotational shank carrying aradially extending thread cutting insert; providing said thread cuttingmilling tool with a secondary radially extending cutter in selectedspaced relationship with said thread cutting insert so as to engage andremove a portion of said work piece along a helical path correspondingwith a helical thread form path corresponding with said thread cuttinginsert.
 2. The method of claim 1 further including the step of spacingapart said thread cutting insert and said secondary cutter on saidelongate rotary shank so as to effect an equivalent linear spacing alongthe axis of said shank between a thread form cutting edge of the threadcutting insert an adjacent edge of said secondary cutter which issubstantially equal to an integer number of thread pitch dimensionsminus one-half a thread pitch dimension.
 3. The method of claim 2further including the step of spacing apart said thread cutting insertand said secondary cutter on said elongate rotary shank so as to effectan equivalent linear spacing along the axis of said shank between athread form cutting edge of the thread cutting insert an adjacent edgeof said secondary cutter which is substantially equal to an integernumber of thread pitch dimensions minus one-half a thread pitchdimension, and also minus an additional selected dimension.
 4. Themethod of claim 3 wherein said additional selected dimension is selectedto be in the range from about 0.002 inch to about 0.010 inch.
 5. Themethod of claim 1 further including the step of disposing said threadcutting insert on said shank with a plurality of thread cutting edgesthereof arranged in an axially extending straight line disposed parallelto the longitudinal axis of said shank, whereby said thread cuttingmilling tool is configured to cut straight screw threads on a workpiece.
 6. The method of claim 1 further including the step of disposingsaid thread cutting insert on said shank with a plurality of threadcutting edges thereof arrange in an axially extending straight linedisposed at a selected angle relative to the longitudinal axis of saidshank, whereby said thread cutting milling tool is configured to cuttapered screw threads on a work piece.
 7. A method of simultaneouslycutting screw threads on a work piece, cutting a threadless portionadjacent an end of the screw threads, and forming a blunt starting endof the screw threads at said threadless portion, said method includingsteps of: providing a thread cutting milling tool comprising an elongaterotary shank defining a longitudinal axis and a working end portion;defining a recess on said working end portion for receiving a radiallyprojecting thread cutting insert providing features on said workingshank portion for also carrying a secondary radially projecting cutterspaced from said thread cutting insert; spinning said thread cuttingtool while simultaneously moving said thread cutting tool in a helicalorbital path relative to the work piece to engage said thread cuttinginset with the work piece to cut screw threads thereon along a helicalthread form path; while spinning and moving said thread cutting millingtool in said helical orbital path relative to the work piece alsoengaging said secondary cutter with the work piece along a respectivehelical orbital path corresponding to said helical thread form path toengage the work piece and remove a selected part thereof.
 8. A fixturefor use in milling screw threads on an upstanding elongate tubular workpiece, said fixture comprising: a base plate having an upstandingtubular boss thereon; a tubular expansible collet member resting uponsaid upstanding tubular boss; a draw bolt extending through said tubularexpansible collet member and through said upstanding tubular boss, saiddraw bolt including a head portion forcefully engageable with an upperend of said tubular expansible collet member to effect expansionthereof; a stripper plate disposed above said base plate, said stripperplate including a bore surrounding said upstanding tubular boss, andsaid stripper plate also providing a surface circumscribing saidupstanding tubular boss and upon which said work piece rests; a movingdevice for vertically reciprocating said stripper plate from a firstposition most closely disposed to said base plate and in which said workpiece is fully engaged upon said tubular expansible collet, and to asecond position spaced from said base plate above said first positionand in which said work piece is engaged by said surface to be elevatedabove full engagement with said tubular expansible collet so as to beeasily removable there from.